Passive Optical Networks

7.2 NETWORK TESTING, CHARACTERIZATION,

AND MONITORING CHALLENGES FOR PON

7.2.1 Fiber’s Key Characteristics

The key characteristic of a fiber link is the power loss that is caused by fiber

attenuation and fiber connections. Attenuation, defined as the ratio of the input

power to the output power, is the loss of optical power as light travels along the

fiber. Attenuation in an optical fiber is caused by absorption, scattering, and

bending losses. The fundamental physical limits imposed on the fiber attenuation

are due to scattering of the silica atoms at shorter wavelengths and the material

absorption at longer wavelengths. There are two minima in the loss curve, one near

1:3 mm and an even lower one near 1: 55 m m. Fiber bending can also induce power

loss because radiation escapes through its bends. The bending loss is inversely

proportional to the bend radius and is wavelength-dependent. It is important to

mention that there may be many fiber macro bends during a PON installation, and

macro bends may have more significant losses at longer wavelengths.

Power loss is also present at fiber connections, such as connectors, splices, and

couplers. Coupling of light into and out of a small-core fiber is much more difficult

to achieve than coupling electrical signals in copper wires since: (i) photons are

weakly confined to the waveguide whereas electrons are tightly bound to the wire,

and (ii) the core of a fiber is typically much smaller than that of an electrical wire.

First, light must be coupled into the fiber from a diverging laser beam, and two fibers

must be connected to each other. Second, connecting two different fibers in a system

must be performed with great care due to the small size of the cores. One wishes to

achieve connections exhibiting: (i) low loss; (ii) low back reflection; (iii) repeatability;

and (iv) reliability. Two popular methods are the permanent splice and the mech-

anical connector. The permanent ‘‘fusion’’ splice can be accomplished by placing

two fiber ends near each other, generating a high-voltage electric arc which melts the

fiber ends, and ‘‘fusing’’ the fibers together. Losses and back reflection are extremely

low being <0.1 dB and <60 dB respectively. Disadvantages are that: (i) a splice is

delicate and must be protected and (ii) it is permanent. Alternatively, there are

several types of mechanical connectors, such as ST, and FC/PC. Losses and back

reflection are still fairly good, and are typically <0.3 dB and <45 dB respectively.

Low loss is extremely important since a light pulse must contain a minimum

amount of power in order to be detected such that ‘‘0’’ or ‘‘1’’ data bit can be

unambiguously detected; thus optical power measurement (or fiber-link loss

measurement) is the most basic testing in any optical-fiber network. Note that

ORL characterization is related to optical power measurement, and is required for

many digital and analog fiber systems, such as PONs. ORL along a fiber span is a

combination of Rayleigh scattering and Fresnel reflections, which can reduce fiber

system performance and increase BER by degrading transmitter stability.

Network Testing, Characterization, and Monitoring Challenges for PON 269